Method and device for cooling and stabilizing strip in a continuous line

ABSTRACT

The invention concerns a method for cooling metal strips using cooling chambers by blowing a gas, in a continuous heat treatment line, wherein: the chambers ( 4,4   a    . . . 4′, 4′   a  . . . ) have a unitary dimension (h) in the moving direction (X) of the strip less than two meters and are split along the direction perpendicular to the moving direction (X) of the strip into a plurality of unitary blowing sectors; each unitary blowing sector is equipped with at least one blowing pressure sensor ( 7 ) and with at least one actuator ( 2 ) for adjusting the pressure of each of said unitary sectors; and a controlling and regulating system controls the actuators ( 2 ) such that the longitudinal theoretical distribution of pressure in the blowing sectors corresponding to a cooling curve of the target strip is adapted so as to take into account a modification of the position of the strip relative to the blowing sectors to avoid any contact thereof with the walls of the devices of the cooling zone without modifying the cooling curve.

The invention relates to lines for the continuous heat treatment of a metal strip, such as annealing lines or metal or organic coating lines.

In these types of lines, the strip is cooled, according to the prior art, using cooling boxes that cool by blowing a gas, for example, a mixture of nitrogen and hydrogen, in a closed cooling chamber of an annealing line or of a galvanizing line upstream of the zinc bath or by blowing air for example in a cooling tower after galvanizing.

The phenomenon of strip instability in the cooling zones is known and is, for example, manifested by a twisting offset of the strip about its longitudinal axis until a stable position is reached, or alternately in the form of torsional oscillations.

It is also a common experience, depending on the type of rolling process, for the edges of the strip to be longer than the center. In this case, the center of the strip is held taut while its edges “float” and may touch one of the surfaces of the cooling boxes when in the extreme positions of their oscillations. It may also happen that the strip has a long center and edges that are held taut, which may result in the strip touching one of the surfaces of the cooling boxes at its center.

The formation of thermally induced folds on the surface of the strip in the cooling zone is a problem regularly faced by the operator of these installations, in particular in the case of wide strip of small thickness. The origin of these folds is also well known. As explained in patent FR 2 802 552 (99/16011) or in patent EP 1 108 795, the formation of folds is caused by slope discontinuity in the cooling curve that generates a compressive stress in the strip. A fold on the surface of the strip is produced if the compressive stress caused by the cooling discontinuity is greater than a limit value.

The object of the invention is to provide a method and a device intended for controlling the position of the strip in a cooling zone in air or in atmosphere, for example, a reducing atmosphere, for the purpose of preventing it from touching the walls of the equipment in the cooling zone, while making the strip follow a theoretical cooling curve corresponding to an intended thermal objective while reducing the risk of fold formation.

A strip is generally cooled by boxes that blow air or a reducing atmosphere through holes or slits fed under pressure by an independent recirculation fan or by a fan common to several boxes. One or more heat exchangers are placed in the circuit, in order to cool the gas after its impact on the strip.

In general, the boxes are identical and fed at a constant pressure.

For a given geometry of the holes or slits and for a blown gas of given composition and temperature, the blowing speed at the holes or slits and therefore the heat exchange between the gas and the strip, and the air pressure exerted by the gas on the strip are directly dependent on the pressure present in the cooling boxes.

In an installation with, for example, three or more cooling boxes that follow one after another in the longitudinal direction of the strip, different cooling curves may occur corresponding to the different pressure settings in the boxes. In a first case, the pressure settings in the three boxes may result in small differences in the cooling slope over the entire duration of the process, thereby preventing the occurrence of folds in the strip. In another case, on the contrary, the cooling of the intermediate box may for example be greater than that of the outermost boxes, thus creating a large difference in cooling slope, which may generate folds in the strip.

Patent FR 2 796 139 (99/08709), or patent EP 1 067 204, discloses a device intended for controlling the position of the strip inside the cooling section in the case of vibration or an offset relative to the central position. This configuration provides settings for the pressures, allowing the blown gas flow rate to be modified along the transverse direction of the strip. This solution allows the supply pressure in the various transverse zones of a given box to be varied, on each side of the strip so as to counteract the rotation of the strip about its longitudinal axis. These adjustments are generally carried out manually by the operators. It frequently happens that the desire to prevent any contact of the strip with the cooling boxes induces fold defects of thermal origin caused by inadequate cooling resulting from inappropriate settings of the cooling distributions. Therefore, despite the presence of transverse setting means of the supply pressures in the boxes, the operators do not often use them, or do not correct a setting deemed to be good, for fear of degrading the situation. Thus, for want of optimally exploiting the available setting means, the thermal or contact defects on the surface of the strip are reduced by reducing the speed of the line or by limiting the width of the strip to be produced, thereby limiting the tonnage produced by the line.

Thus, the methods and devices according to the prior art cannot simultaneously and effectively control the cooling curve of the strip and keep said strip in position in a cooling zone. In addition, owing to imperfections in this prior art, these methods and devices, if they are not implemented correctly may induce defects in the products.

The proposed invention provides a solution to this problem.

According to the invention, a method of cooling a metal strip by cooling boxes that cool by blowing a gas, in particular air or a mixture consisting of nitrogen and hydrogen, in a continuous heat treatment line, is characterized in that:

-   -   the boxes have a unitary dimension in the run direction of the         strip of less than two meters and are divided in the direction         perpendicular to the run direction of the strip into a plurality         of unitary blowing sectors;     -   each unitary blowing sector is equipped with at least one         actuator for adjusting the pressure of each of these unitary         sectors; and     -   a control/regulation system controls the actuators in such a way         that the theoretical longitudinal pressure distribution in the         blowing sectors corresponding to an intended strip cooling curve         is adapted so as to take into account a change in the position         of the strip relative to the blowing sectors, so as to avoid any         contact of said strip with the walls of the equipment in the         cooling zone without modifying the cooling curve.

Thus, the theoretical longitudinal pressure distribution in the successive blowing sectors in the run direction of the strip corresponding to an intended strip cooling curve is adapted so as to take into account a change in the position of the strip relative to the blowing sectors so as to avoid any contact of said strip with the walls of the equipment in the cooling zone without modifying the cooling curve.

The desired theoretical cooling curve does not have a break in slope.

Each unitary blowing sector may be equipped with at least one sensor for measuring the blowing pressure and with at least one actuator for adjusting the pressure in each of these unitary sectors; and

-   -   the information coming from the sensors is sent to the         control/regulation system that controls the actuators in such a         way that the theoretical longitudinal pressure distribution in         the blowing sectors corresponding to an intended strip cooling         curve is adapted so as to take into account a change in the         position of the strip relative to the blowing sectors so as to         avoid any contact of said strip with the walls of the equipment         in the cooling zone without modifying the cooling curve.

As a variant, each unitary blowing sector may be equipped with at least one device for measuring the blowing flow rate and at least one actuator for adjusting the pressure in each of these unitary sectors; and

-   -   the information coming from the devices for measuring the         blowing flow rate is sent to the control/regulation system that         controls the actuators in such a way that the theoretical         longitudinal pressure distribution in the blowing sectors         corresponding to an intended strip cooling curve is adapted so         as to take into account a change in the position of the strip         relative to the blowing sectors so as to avoid any contact of         the latter with the walls of the equipment in the cooling zone         without modifying the cooling curve.

Preferably, each box is divided into at least two, and advantageously three, unitary blowing sectors over the width of the strip.

According to one exemplary embodiment of the invention, the control/regulation system is programmed so as:

-   -   in a first step, to define, per “longitudinal slice”, the         pressure setpoints for each box according to the cooling curve         to be obtained; and     -   in a second step, if a correction to the position of the strip         is demanded and depending on the type of correction introduced,         manually or automatically, to modify the pressure distribution         in a given longitudinal “slice” of the cooler so as to obtain         the desired strip position correction while keeping the selected         cooling curve intact.

A “longitudinal slice” is the set of blowing sectors located on either side of the strip along the cooling zone in the run direction of the strip and placed at the same distance from the axis of the strip.

According to one exemplary embodiment, the control/regulation system can take into account a manual setpoint or an automatic setpoint for setting the pressure of one or more blowing sectors.

If a change in pressure is required, manually or automatically, in one or more blowing sectors in order to correct the position of the strip, the control/regulation system adjusts the pressure settings in the other blowing sectors so that the strip is placed in a position that avoids any contact of the latter with the walls of the equipment in the cooling zone and that, at any point on the strip, the temperature follows a desired theoretical cooling curve.

The pressures adjusted on either side of the strip so as to correct the position of the strip are defined so that their resultant achieves the overall thermal objective defined for the section in question and over the entire width of the strip, depending on the desired theoretical cooling curve.

In one exemplary embodiment of the invention, the automatic control/regulation system may determine:

-   -   in a first step, according to the theoretical cooling curve, the         characteristics of the strip and the data on the entire         installation, an overall cooling power for the pairs of two         unitary cooling sectors located on either side of a given zone         of the strip; and     -   in a second step, according to the desired position of the strip         in the zone in question, the blowing pressures for the two         unitary sectors of each pair, which pressures, while providing         the desired overall cooling, may be different so as to adjust         the position of the strip.

The pressures in the unitary sectors of several boxes located on one side of the strip may be increased or decreased simultaneously so that a correction to the position of the strip parallel to itself is applied.

The pressures adjusted so as to correct the position of the strip are defined so that their resultant corresponds to the overall thermal objective defined for the section in question and over the entire width of the strip according to the desired theoretical cooling curve.

According to another option, the pressures in the unitary sectors of a given level located on each side of the strip are adjusted so as to apply a correction to the position of the strip as a twist about its main axis.

According to yet another option, for holding the strip in position, an alternating setting of the pressure along the run direction of the strip is provided in the cooling boxes, with a higher pressure in one box followed by a lower pressure in the next box placed on the same side as the strip, and a higher pressure in one box corresponds to a lower pressure in the box facing it on the other side of the strip so as to produce an alternating deformation of the strip.

Thus, by implementing the invention it is possible to apply a “multiprogram” correction to the position of the strip without modifying the thermal cooling cycle applied to the strip. The correction may be introduced manually into the system or may be controlled by strip position sensors in the furnace.

The setpoints for setting the pressure in the blowing sectors may be delivered by a computer on the basis of a thermomechanical model that takes into account the nature of the material of the strip and the heat treatment to be applied to the strip.

The algorithm for controlling the cooling curve and for stabilizing the position of the strip may use fuzzy logic and/or neural systems.

The invention also relates to a device for cooling a metal strip in a continuous heat treatment line, comprising cooling boxes which cool by blowing a gas, in particular air or a mixture consisting of nitrogen and hydrogen, which follow one after another in the run direction of the strip, characterized in that:

-   -   the boxes have a unitary dimension in the run direction of the         strip of less than two meters and are divided in the direction         perpendicular to the run direction of the strip into a plurality         of unitary blowing sectors;     -   each unitary blowing sector is equipped with at least one         actuator for adjusting the pressure in each of these unitary         sectors; and     -   a control/regulation system is provided, which system controls         the actuators in such a way that the strip is placed in a         position that avoids any contact of said strip with the walls of         the equipment in the cooling zone and that, at any point on the         strip, the temperature follows a desired theoretical cooling         curve.

Each blowing sector may be equipped with at least one sensor for measuring the blowing pressure and/or with a device for measuring the blowing flow rate, and the information coming from the sectors and/or from the blowing flow rate measurement devices is sent to the control/regulation system.

Preferably, the boxes have a unitary dimension in the run direction of the order of one meter, and each box is divided into at least two unitary blowing sectors over the width of the strip for a right/left correction, or at least three sectors for a center/edge correction.

The control/regulation system is designed to adjust all the pressures in the unitary sectors of the cooling zone according to a given pressure map in the directions parallel and perpendicular to the run direction of the strip by the choice of setpoints introduced into the system in such a way that the settings obtained are adapted according to the nature of the strip and to the transverse profile of the strip upon entering the cooling section.

The control/regulation system is designed to make, when necessary, for example on the basis of manual setpoints introduced into the system, an adjustment to all the pressures in the boxes in the cooling zone, which results mainly in a correction to the position of the strip as a twist about its main axis.

The control/regulation system is also designed to make, when necessary, for example on the basis of manual setpoints introduced into the system, an adjustment to all of the pressures in the boxes of the cooling zone, which results mainly in a correction to the position of the strip so as to produce an alternating deformation of the strip along its longitudinal direction.

The control/regulation system is programmed so as to determine:

-   -   in a first step, according to the theoretical cooling curve,         overall data, and characteristics of the strip, an overall         cooling power for each of the pairs of two unitary cooling         sectors located on either side of a given zone of the strip; and     -   in a second step, according to the desired position of the strip         in the zone in question, the blowing pressures for the two         unitary sectors of each pair, which pressures while still         providing the desired overall cooling, may be different in order         to adjust the position of the strip.

The invention consists, apart from the abovementioned arrangements, of a number of other arrangements which will be explained in greater detail below with regard to exemplary embodiments described with reference to the appended drawings, which embodiments are in no way limiting. In these drawings:

FIG. 1 is a vertical sectional diagram of a strip cooling device according to the invention;

FIG. 2 is a curve showing the variation in temperature of the strip plotted on the Oy axis as a function of the position in the device of FIG. 1 plotted on the Ox axis;

FIG. 3 is a partial schematic view of the device and of the strip on the line III-III in FIG. 1;

FIG. 4 is a vertical sectional diagram of a variant of the strip cooling device;

FIG. 5 is a curve showing the variation in temperature of the strip plotted on the Oy axis as a function of the position in the device of FIG. 4 plotted on the Ox axis;

FIG. 6 is a diagram in horizontal section of a variant of the strip cooling device;

FIG. 7 is a diagram in vertical section of another variant of the strip cooling device;

FIG. 8 illustrates curves showing the variation in the temperature of the strip plotted on the Oy axis as a function of the position in the cooling device plotted on the Ox axis;

FIG. 9 is a diagram in horizontal section of another variant of the strip cooling device;

FIG. 10 is a diagram in vertical section of another variant of the strip cooling device;

FIG. 11 is a diagram in vertical section of another variant of the strip cooling device; and

FIG. 12 is a diagram in vertical section on a smaller scale, of a variant of the device shown in FIG. 4.

The drawings, especially FIGS. 1, 3, 4 and 6, show a device for cooling a metal strip 1 that runs vertically from the top down along the arrow X in the example in question. This example is not limiting and the run direction could instead be from the bottom up or in a direction other than the vertical, especially an oblique direction.

The cooling device comprises, as shown schematically in the drawings, on either side of the strip 1, boxes 4, 4 a, 4 b . . . 4′, 4′a, 4′b . . . for blowing air or a reducing atmosphere through holes t or slits fed under pressure by an independent recirculation fan 2 specific to each box or by a fan common to several boxes, as shown in FIG. 4. One or more heat exchangers 3 are placed in the circuit so as to cool the gas after its impact on the strip. In general, the boxes are fed at constant pressure. The boxes 4, 4 a, 4 b . . . 4′, 4′a, 4′b . . . follow one after another in the run direction X of the strip.

According to one exemplary embodiment of the invention, shown in FIG. 11, the boxes 4, . . . , 4′, . . . located on either side of the strip are not directly opposite one another but offset in the run direction of the strip by a fraction of their length.

FIG. 12 illustrates an alternative embodiment of the cooling tower of a galvanizing line shown in FIG. 4. Although in FIG. 4 a return circuit to the intake side of the fan 2 is provided, in FIG. 12 there is no such circuit for returning the blown air back to the intake side of the fan 2.

In general, a box 4, 4 a, . . . 4′, 4′a, . . . extends over the entire width of the strip. However, it is possible to juxtapose, horizontally, several boxes of smaller width than that of the strip, the total width of the boxes covering the width of the strip.

For a given geometry of holes or slits and for a blown gas of given composition and temperature, the blowing speed through the holes or slits and therefore the heat exchange between the gas and the strip and the pressure exerted by the gas on the strip are directly dependent on the pressure in the cooling boxes 4, 4 a, . . . 4′, 4′a, . . . .

According to the invention, the blowing boxes 4, 4 a, . . . 4′, 4′a, . . . are divided up in the direction perpendicular to the run direction X of the strip into a plurality of unitary blowing sectors. Each box has, along the run direction X of the strip, a small unitary dimension h, that is to say less than two meters, preferably of the order of (close to or equal to) one meter and comprises several blowing nozzles or rows of holes.

Each box is divided into at least two, preferably three or five, unitary blowing sectors over the width of the strip. Each sector is equipped with at least one pressure sensor 7 and with at least one actuator, for example in the form of a control valve 6 or a similar member. As a variant, for each unitary sector, the actuator may be an independent fan 2, as illustrated in FIG. 1, the rotation speed of the turbine of which is controlled by a variable speed drive so as to obtain the desired pressure. The actuators 6, 2 make it possible to adjust the pressure in each of the unitary sectors in directions parallel and perpendicular to the run direction of the strip.

To complement the pressure sensor 7, it is also possible to integrate, into each blowing sector, a probe for measuring the temperature of the blown gas.

It is also possible to provide for each unitary blowing sector, instead of or as a complement to the pressure sensor, a device 8 (FIG. 1) for measuring the blowing flow rate on the delivery side of the fan 2. In FIG. 1, such a measuring device 8 has been shown only on the top and immediately lower fans. Of course, such a measuring device may be provided on each independent fan 2.

These pressure, temperature and/or flow rate measurements are sent to a control/regulation system R (FIGS. 3 and 6) which adjusts the turbine rotation speed of the fan 2 or the position of the pressure regulator 6 so as to adjust the pressure in each blowing sector according to a theoretical or calculated cooling curve that depends for example on the nature of the material to be treated or on the type of heat treatment cycle desired.

Thus, a veritable meshing (FIG. 3) of the delivery of cooling on each side of the strip is formed, this meshing allowing the delivery of cooling to be extremely precisely controlled in both the longitudinal X and transverse T directions of the strip. The meshing is physically represented by a matrix of blowing sectors, facing each side of the strip, in horizontal rows corresponding to the boxes 4, 4 a, 4 b, 4 c . . . 4′, 4′a, . . . and vertical columns α, β, γ . . . corresponding to the sectors of each box. A unitary sector will be denoted by the reference 4, 4 a, . . . or 4′, 4′a . . . , of the box, depending on which side of the strip 1, followed by the Greek letter of the column, for example, the sector 4 aβ in FIG. 3. A horizontal row generally corresponds to a single box.

The sensors 7 and actuators 6 are connected to the control/regulation system R.

Shown in FIG. 5, to the right of FIG. 4, are two examples of cooling curves C and D obtained with the boxes 4 of short unitary length, of the order of one meter. FIG. 5 shows it is possible to produce continuous cooling curves that do not generate folds, with very different profiles depending on the desired theoretical cooling curve.

The control/regulation system R controls the actuators 6, 2 so that, at any point on the strip, the temperature follows a desired theoretical cooling curve but does not have a break in slope, incorporating the strip position correction introduced manually or depending on the information received from a strip position sensor on the line so that the strip is held in a position that avoids any contact between said strip and the walls of the equipment in the cooling zone.

The pressure and temperature measurements allow the blowing speeds in each sector to be adjusted so that there is no difference in fluid flow action on the various sections of the strip that could run the risk of deforming the strip, or, on the contrary, in such a way as to create a controlled difference in fluid flow action on the strip enabling an incorrectly positioned strip to be corrected.

The system R comprises a computer, which determines:

-   -   on the basis of the desired theoretical cooling curve, initial         setting of the matrix by assigning the required pressures to the         blowing sectors so as to obtain the desired cooling at a point;         and     -   following this setting, an adjustment is made to the pressure         settings for the blowing sectors located on either side of the         strip in such a way as to stabilize the strip and prevent any         contact of said strip with the walls of the equipment in the         cooling zone, without deviating from the intended thermal curve.

This adjustment to the pressure settings on either side of the strip is possible, without being prejudicial to the desired cooling curve being followed, since the same overall cooling of a zone in question of the strip may be obtained:

-   -   either with an identical blowing pressure on either side of the         strip which, in this case, is not stressed so as to move it         transversely to its run direction X;     -   or with a higher blowing pressure on one side of the strip than         on the other, the strip in this case being stressed so as to         move transversely to its run direction X.

In other words, the control system R is programmed, with suitable software, so as to determine:

-   -   in a first step, according to the theoretical cooling curve,         data relating to the overall device and the installation and         characteristics of the strip 1, in particular its entry         temperature and its composition, an overall cooling power for         the pairs of two unitary cooling sectors located on either side         of a given zone of the strip; and     -   in a second step, according to the selected correction to the         position of the strip in the zone in question, the blowing         pressures for the two unitary sectors of each pair, which         pressures, while still providing the desired overall cooling,         may be different in order to adjust the position of the strip.

According to one exemplary embodiment of the invention, the control system R is programmed to treat the unitary cooling sectors per longitudinal “slice” parallel to the run direction of the strip.

FIG. 9 shows in detail the method of controlling the cooling, with action to compensate for a twist deformation of the strip, without the formation of folds on the strip. The temperature graph shown in FIG. 8 includes, indicated by the solid line, the theoretical curve F of the treatment to be carried out on the strip, for example, depending on the nature of the metallurgical treatment to be applied to it.

Derived from this curve F is the efficiency, or power, of the heat exchange for each pair of unitary sectors located on either side of the strip at a given position along the length of the cooling zone (in the same horizontal row) according to the temperature of the blown gas. The control system R will generate a pressure setpoint for each box 4, 4 a, . . . 4′, 4′a, . . . and for each unitary sector 4α, 4β, . . . 4 aα, 4β, etc. of this box. This pressure setpoint could be used to control the rotation speed of each fan 2 or the position of the pressure-regulating valve 6. The pressure setpoints for the various blowing sectors make it possible to obtain not only the desired cooling but also the positioning of the strip.

FIG. 9 illustrates an example of shift of the strip in one cooling section and the actions that will be undertaken in order to remedy this. In particular, the system R causes the pressure in the bottom-left sector 4′aβ and top-right sector 4 aα in FIG. 9 to be reduced and for the blowing pressure in the top-left sector 4′aα and bottom-right sector 4 aβ to be increased so as to correct the position of the strip, while still providing the desired cooling. It will be understood that this principle can be applied to any type of division of the boxes on the transverse direction of the strip.

The system R for controlling and regulating the cooling section will, on demand by the operator or depending on reception of information from a sensor for determining the position of the strip on the line, recalculate the pressure setpoints for each part of each box at each level of the cooling zone so as to obtain the pressure curves for the “+” sectors and “−” sectors that correspond to curves E and G in FIG. 8.

FIG. 8 shows that the action of compensating for the deformation of the strip does not incur a particular risk of folds since the “+−” curve E and the “−+” curve G do not have a singularity of the kind to produce these folds and that the sum of the “+” and “−” cooling actions on the two faces of the strip at each point on the curve is substantially in accordance with the initially defined theoretical curve F.

In this way, it is possible for the cooling on one face of the strip to be continuously reduced, and proportionally increased on its other face over the length of the cooling zone according to curves E and G so as to move the strip parallel to itself so as to eliminate any risk of contact with the blowing boxes.

FIG. 10 shows another means for correcting the position of the strip between the items of equipment in the cooling zone by an alternating setting, along the run direction of the strip, of the pressure in the cooling boxes with a higher (+) pressure in a box 4 a followed by a lower pressure in the next box 4 b placed on the same side of the strip, and a higher (+) pressure in a box 4 aα, 4′bα corresponds to a lower pressure on the box 4′a, 4 b facing it on the other side of the strip, so as to produce an alternating deformation of the strip, of sinusoidal appearance, along its longitudinal direction.

The invention also allows all the pressures in the boxes of the cooling zone to be adjusted according to a given pressure map along directions parallel and perpendicular to the run direction of the strip by choosing a manual setpoint so that the settings obtained are adapted to the nature of the strip and to the transverse profile of the strip upon entering the cooling section. For example, a first setpoint will be adapted to a strip having edges longer than the center and a second setpoint will be adapted to a strip with a long center.

Adjusting all the pressures in the boxes and in their unitary sectors within the cooling zone results for example mainly in:

-   -   a correction to the position of the strip 1 parallel to itself;     -   or to a correction to the position of the strip 1 as a twist         about its main axis;     -   or to a center/edges correction, with a different correction for         each edge;     -   or to a correction to the position of the strip 1 so as to         produce an alternating deformation of the strip;     -   or else, by combining various correction principles, such as,         for example a twist plus a correction parallel to the strip.

According to one exemplary embodiment of the invention, the setpoints for setting the pressure in the blowing sectors are obtained by a computer on the basis of a thermomechanical model that takes into account the nature of the material of the strip and the heat treatment to be applied to the strip. The algorithm for controlling the cooling curve and for stabilizing the position of the strip uses for example fuzzy logic and/or neural systems.

The method of the invention therefore enables the cooling pressures to be adjusted over the entire length of the cooling section according to a theoretical curve or optimum practical curve, without the risk of folds appearing or with a minimum risk thereof, and to do so by correcting an error in the position of the strip by incorporating manual setpoints or setpoints derived from a position sensor in terms of shape or twist without incurring an additional risk of folds and without reducing the production capability of the line. 

1. A method of cooling a metal strip by cooling boxes that cool by blowing a gas, in particular air or a mixture consisting of nitrogen and hydrogen, in a continuous heat treatment line, wherein: the boxes (4, 4 a . . . 4′, 4′a . . . ) have a unitary dimension (h) in the run direction (X) of the strip of less than two meters and are divided in the direction perpendicular to the run direction (X) of the strip into a plurality of unitary blowing sectors (4α, 4β, 4γ . . . 4 aα, 4 aβ, 4 aγ . . . ; 4′α, 4′β, 4′γ . . . 4′aα, 4′aβ, 4′aγ . . . ); each unitary blowing sector is equipped with at least one actuator (6; 2) for adjusting the pressure of each of these unitary sectors; and a control/regulation system (R) controls the actuators (6; 2) in such a way that the theoretical longitudinal pressure distribution in the blowing sectors corresponding to an intended strip cooling curve (F) is adapted so as to take into account a change in the position of the strip relative to the blowing sectors, so as to avoid any contact of said strip with the walls of the equipment in the cooling zone without modifying the cooling curve (F).
 2. The method as claimed in claim 1, wherein: each unitary blowing sector is equipped with at least one sensor (7) for measuring the blowing pressure and with at least one actuator (6; 2) for adjusting the pressure in each of these unitary sectors; and the information coming from the sensors (7) is sent to the control/regulation system (R) that controls the actuators (6; 2) in such a way that the theoretical longitudinal pressure distribution in the blowing sectors corresponding to an intended strip cooling curve (S) is adapted so as to take into account a change in the position of the strip relative to the blowing sectors so as to avoid any contact of said strip with the walls of the equipment in the cooling zone without modifying the cooling curve (F).
 3. The method as claimed in claim 1, wherein: each unitary blowing sector is equipped with at least one device (8) for measuring the blowing flow rate and at least one actuator (6; 2) for adjusting the pressure in each of these unitary sectors; and the information coming from the devices (8) for measuring the blowing flow rate is sent to the control/regulation system (R) that controls the actuators (6; 2) in such a way that the theoretical longitudinal pressure distribution in the blowing sectors corresponding to an intended strip cooling curve (F) is adapted so as to take into account a change in the position of the strip relative to the blowing sectors so as to avoid any contact of the latter with the walls of the equipment in the cooling zone without modifying the cooling curve (F).
 4. The method as claimed in claim 1, wherein each box (4, 4 a . . . , 4′, 4′a . . . ) is divided into at least two unitary blowing sectors over the width of the strip for a right/left correction, or at least three blowing sectors for a center/edge correction.
 5. The method as claimed in claim 1, wherein the control/regulation system is programmed so as: in a first step, to define, per “longitudinal slice”, the pressure setpoints for each box (4, 4 a . . . 4′, 4′a . . . ) according to the cooling curve to be obtained; and in a second step, if a correction to the position of the strip (1) is demanded and depending on the type of correction introduced, manually or automatically, to modify the pressure distribution in a given longitudinal “slice” of the cooler so as to obtain the desired strip position correction while keeping the selected cooling curve intact.
 6. The method as claimed in claim 1, wherein the control/regulation system (R) takes into account setpoints for setting the pressure in one or more blowing sectors so as to adjust the pressure settings in the other blowing sectors so that the strip (1) is placed in a position that avoids any contact of the latter with the walls of the equipment in the cooling zone and that, at any point on the strip, the temperature follows a desired theoretical cooling curve (F).
 7. The method as claimed in claim 1, wherein the pressures adjusted on either side of the strip so as to correct the position of the strip (1) are defined so that their resultant achieves the overall thermal objective defined for the section in question and over the entire width of the strip, depending on the desired theoretical cooling curve.
 8. The method as claimed in claim 1, wherein the control/regulation system (R) determines: in a first step, according to the theoretical cooling curve (F), the characteristics of the strip (1) and the data on the entire installation, an overall cooling power for the pairs of two unitary cooling sectors located on either side of a given zone of the strip; and in a second step, according to the desired position of the strip in the zone in question, the blowing pressures for the two unitary sectors of each pair, which pressures, while providing the desired overall cooling, may be different so as to adjust the position of the strip.
 9. The method as claimed in claim 1, wherein the pressures in the unitary sectors (4α, 4β, . . . 4 aα, 4′aβ . . . ) of several boxes located on one side of the strip are adjusted simultaneously, namely increased or decreased, so as to apply a correction to the position of the strip (1) parallel to itself.
 10. The method as claimed in claim 1, wherein the pressures in the unitary sectors of a given level (4α, 4β, . . . 4 aα, 4 aβ . . . ) located on each side of the strip are adjusted so as to apply a correction to the position of the strip (1) as a twist about its main axis.
 11. The method as claimed in claim 1, wherein for holding the strip in position, an alternating setting of the pressure along the run direction of the strip (1) is provided in the cooling boxes, with a higher pressure in one box followed by a lower pressure in the next box placed on the same side as the strip, and a higher pressure in one box corresponds to a lower pressure in the box facing it on the other side of the strip so as to produce an alternating deformation of the strip.
 12. The method as claimed in claim 1, wherein the setpoints for setting the pressure in the blowing sectors is delivered by a computer on the basis of a thermomechanical model that takes into account the nature of the material of the strip and the heat treatment to be applied to the strip.
 13. The method as claimed in claim 12, wherein the algorithm for controlling the cooling curve and for stabilizing the position of the strip uses fuzzy logic and/or neural systems.
 14. A device for cooling a metal strip in a continuous heat treatment line, comprising cooling boxes (4, 4 a . . . , 4′, 4′a . . . ) which cool by blowing a gas, in particular air or a mixture consisting of nitrogen and hydrogen, which follow one after another in the run direction (X) of the strip, wherein: the boxes (4, 4 a . . . , 4′, 4′a . . . ) have a unitary dimension (h) in the run direction (X) of the strip of less than two meters and are divided in the direction perpendicular to the run direction (X) of the strip into a plurality of unitary blowing sectors (4α, 4β, . . . 4 aα, 4 aβ, . . . ; 4′α, 4′β, 4′aα, 4′aβ, . . . ); each unitary blowing sector is equipped with at least one actuator (6; 2) for adjusting the pressure in each of these unitary sectors along directions parallel and perpendicular to the run direction of the strip; and a control/regulation system (R) is provided, which system controls the actuators (6; 2) in such a way that the strip is placed in a position that avoids any contact of said strip with the walls of the equipment in the cooling zone and that, at any point on the strip, the temperature follows a desired theoretical cooling curve (F).
 15. The device as claimed in claim 14, wherein each blowing sector is equipped with at least one sensor (7) for measuring the blowing pressure and with at least one actuator (6; 2) for adjusting the pressure in each of these unitary sectors, and the information coming from the sectors (7) is sent to the control/regulation system (R) that controls the actuators (6; 2) in such a way that the theoretical longitudinal pressure distribution in the blowing sectors corresponding to an intended strip cooling curve (F) is adapted so as to take into account a change in the position of the strip relative to the blowing sectors so as to avoid any contact of said strip with the walls of the equipment in the cooling zone without modifying the cooling curve (F).
 16. The device as claimed in claim 14, wherein each unitary blowing sector is equipped with at least one device (8) for measuring the blowing flow rate and with at least one actuator (6; 2) for adjusting the pressure in each of these unitary sectors, and the information coming from the devices (8) for measuring the blowing flow rate is sent to the control/regulation system (R) that controls the actuators (6; 2) in such a way that the theoretical longitudinal pressure distribution in the blowing sectors corresponding to an intended strip cooling curve (F) is adapted so as to take into account a change in the position of the strip relative to the blowing sectors so as to avoid any contact of said strip with the walls of the equipment in the cooling zone without modifying the cooling curve (F).
 17. The device as claimed in claim 14, wherein each box is divided into at least two unitary blowing sectors over the width of the strip for a right/left correction, or at least three blowing sectors for a center/edge correction.
 18. The device as claimed in claim 14, wherein the control/regulation system (R) is designed to adjust all the pressures in the unitary sectors of the cooling zone according to a given pressure map in the directions parallel and perpendicular to the run direction of the strip by the choice of setpoints introduced into the system (R) in such a way that the settings obtained are adapted according to the nature of the strip and to the transverse profile of the strip upon entering the cooling section.
 19. The device as claimed in claim 14, wherein the control/regulation system (R) is designed to make, when necessary, an adjustment to all the pressures in the boxes in the cooling zone, which results mainly in a correction to the position of the strip (1) parallel to itself.
 20. The device as claimed in claim 14, wherein the control/regulation system (R) is designed to make, when necessary, an adjustment to all of the pressures in the boxes of the cooling zone, which results mainly in a correction to the position of the strip (1) as a twist about its main axis.
 21. The device as claimed in claim 14, wherein the control/regulation system (R) is designed to make, when necessary, an adjustment to all of the pressures in the boxes of the cooling zone, which results mainly in a correction to the position of the strip (1) so as to produce an alternating deformation of the strip along its longitudinal direction.
 22. The device as claimed in claim 14, wherein the automatic control/regulation system (R) is programmed so as to determine: in a first step, according to the theoretical cooling curve (F), overall data and characteristics of the strip (1), an overall cooling power for each of the pairs of two unitary cooling sectors located on either side of a given zone of the strip; and in a second step, according to the desired position of the strip in the zone in question, the blowing pressures for the two unitary sectors of each pair, which pressures while still providing the desired overall cooling, may be different in order to adjust the position of the strip. 